The invention relates to a microscope stand for a wafer inspection microscope.
Inspection microscopes are used in the semiconductor industry to check the fabrication stages of wafers and/or semiconductor modules. They mostly have the capacity to feed the wafer automatically from wafer storage cassettes. In this case, for the purpose of each microscopic investigation an individual wafer is automatically respectively extracted from a cassette and, after investigation, redeposited therein or in another cassette.
In wafer production, a plurality of fabrication and test steps are arranged in close proximity to one another spatially in so-called clusters in the production area, in order to utilize the cost intensive clean room area optimally. The wafers investigated and processed are passed on from one processing step to the next, without being redeposited in the storage cassettes provided for them. The arrangement of the individual operations in a cluster is classically next to one another downstream of a wall. Provided directly downstream of the wall is a type of conveying belt for the wafers, from which the wafers can be removed for the individual operations and redeposited there. The individual fabrication and test units are arranged downstream of the transportation system and arranged downstream of this, in turn, the work stations for the operating staff. This requires it to be possible for each processing unit, or else test unit to be arranged in a row and be loaded from behind.
In the previously known classic microscopes of so-called "C" design, the stand base, the pillar and the crosshead form a "C" open toward the user in side view. The generically determinative DE 196 01 731 A1 describes such a microscope. In front view, the crosshead is arranged, with assigned objective, at a distance above the stand base with assigned condenser, and the pillar is arranged behind these two. The stand base bears the object stage which, as seen by the observer, is accessible from the left, right and front for the user. However, it is not possible to load the object stage with samples from behind.
The MX50 microscope from Olympus also has a "C stand". Details on this microscope are to be found in the company brochure "Halbleiter-InspektionsMikroskop MX50", ["MX50 semiconductor inspection Microscope"] Art. No. 30798. A feeder station is connected on the left next to the microscope. As a rule, such a feeder station comprises a wafer storage cassette for the wafers to be investigated, and at least one further one for the investigated wafer, as well as a handling system for transporting the wafers between the storage cassettes and the object stage. The wafer feed to the object stage of the microscope is performed in this case from the left. The storage cassettes with the wafers need to be inserted manually into the feeder and also re-extracted manually.
This contradicts the customary processing procedure in a cluster, in which the individual wafers are automatically fed from the rear side to the test stations from a conveying belt and, after testing, are also redelivered to the conveying belt after there [sic]. However, the wafer conveying belt and the feeder station must be adapted to one another as far as possible with connecting elements in order to incorporate the previously known inspection microscope with feeder station in the cluster operation. In this case, it is not always possible to arrange the inspection microscope with the feeder in the row next to the other investigating units in the cluster. In order to incorporate it in the other process steps, it is frequently necessary to place it obliquely into the room, with the result that it projects into the narrow passage for the work stations of the operating staff. This additionally requires cost-intensive clean room area.
Such an adaptation is frequently not even possible. The point is that the manual wafer supply of the feeder with wafer cassettes remains as before, and therefore necessitates regular staff intervention, which constitutes an even greater disadvantage for operation.
Thus, the disadvantage of the previously known classic microscope stand of "C" design consists in that it is not possible for the microscope object stage to be loaded from behind, as is required in the semiconductor industry.
U.S. Pat. No. 4,582,191 therefore describes as a supplement to these classic microscopes a special annular transportation and handling system which is arranged at a spacing around the pillar. A stationary standing ring provided with slide rails is borne by an angle piece which is screwed to the pillar. Movably arranged on the slide rails is a second, concentric transportation ring which is fitted with wafer holding devices. The wafers are fed to this annular transportation system from a storage cassette via a lateral assembly line device, and moved around the pillar by motor. It follows that the described transportation system can also be used to guide wafers from the rear side of the column around the latter to the front side. There, the wafer is handed over to the object stage, on which there is mounted in addition a special height adapter with a wafer vacuum holder.
A disadvantage of the system described consists in that the annular transportation system is fastened on the pillar. A microscope of the performance class required for the investigations mentioned is a very sensitive unit which is extremely accurately tuned in design with reference to a low vibration response and high quality statics. Thus, any mass not provided by the manufacturer and additionally attached such as, for example, the annular transportation system, influences the balanced statics and thereby impairs the imaging behavior of the microscope. Furthermore, the vibrations of the drive motor of the annular transportation system can be transmitted disturbingly to the stand.
Moreover, with its motor and its guide elements sliding on one another, the annular transportation system additionally produces undesired contaminations in the clean rooms. Owing to its dimensions, the annular transportation system also additionally enlarges the floor space of the microscope, and thereby lays claim to expensive floor area of the clean room.
The problem of incorporating inspection microscopes in the fabrication lines of the semiconductor industry is therefore an urgent one, even in the case of a modern inspection microscope--such as that named by way of example--and for the use of auxiliary transportation systems, since the outlay on conversion and adaptation measures, a high usage of space on the clean room floor area, contaminations of the clean room, uneconomic work cycles and a high level of staff use impinge directly on the production costs.
It is therefore the object of this invention to specify a microscope stand with a small footprint for a wafer inspection microscope which ensures increased freedom in the loading of the object stage with large-area wafers. It is intended not to introduce any additional contaminations into the clean room. Moreover, the novel stand is also to permit investigations on large-area objects, in order to satisfy future demands from the semiconductor industry with reference to investigation on large-area wafers.